If you go down to Fleet Street today, you're sure of not so much a big surprise, as a stark warning from the future. It is here that Sainsbury has chosen to trial its Fresh Kitchen, a spin-off takeaway offering hot 'n' cold food which, industry analysts predict, will soon be rolled out nationwide. The grocery giant, like several other British supermarkets, is determined to take on the nation's sandwich shops and fast food chains, head on.

If the idea of such diversification distresses you, it is nothing to actually visiting Fresh Kitchen. Spartan, cheaply utilitarian in design, the Fleet Street prototype feels like a cross between Gregg's, McDonald's and an emergency field kitchen. It is loud, it is brightly lit, it is packed with people (there is token, very limited seating), and the counter staff and cashiers have clearly been trained to process each sale in seconds. They shout for the next customer, shout out the orders. Shout them out again. Basically, there is a lot of shouting.

Naturally, prices are competitive. According to this report (pdf), Fresh Kitchen is selling the cheapest coffee on the street. Sandwiches "freshly made here today" (can that really still be a selling point in 2011?) run from a half "skinny" chicken sandwich (£1.39) to baguettes at £2.79, with many staple sandwiches coming in at around or just under £2.

But what of the quality? Swerving a £3.99 hot meat baguette (the pork and beef on the counter looked dry and shrivelled) I opted, instead, for lasagne, one of several, "hot meals, freshly cooked here today". Confused as to what exactly that meant, I asked the Sainsbury press office to explain: "We prepare our hot meals from a selection of components, which are cooked on site and served hot to order. The components themselves are [sourced] from across our supplier base ... The lasagne is prepared in our kitchen from pasta, bolognaise and béchamel sauce components. [It is] definitely not a pre-portioned ready meal."

I couldn't, if you'll pardon the pun, taste the difference. The lasagne was every bit as joyless as your average supermarket ready-meal. It arrived at the authentic solar temperature, the lasagne sheets pallid and glutinous, the minced beef in the rumoured-to-be tomato sauce like curiously spongy grit. It came with an undressed side "salad" of lettuce leaves and, apart from the thin layer of cheese on top, it tasted of almost nothing. It made me, as much as food ever can do, angry.

I dropped in at Fresh Kitchen while researching a budget eats piece on central London, which meant I was in the process of eating good food at similar prices (Fresh Kitchen hot meals are £4.49 each) all over the capital. I wanted to stand outside and redirect anyone buying soup (£2.49) to the nearby independent Cafe Below, where I later enjoyed a fantastic, vibrant minestrone that cost just 21p more.

There is an argument that, as the perceived purveyors of "good" food, Waitrose and M&S will bring high standards to the table (though the grab-and-go food offered at M&S Simply Food or by Waitrose at Boots adds just another layer of bland mediocrity). Certainly, the M&S Kitchen sites appear to be designed to deliver relatively wholesome, honest food, cooked to order by proper chefs.

Long-term, however, running decent restaurants and cafes in any category is a complex, expensive business, and even the most casual has to differentiate itself. Starbucks, for instance, flatters those who consider themselves expert in coffee. Pizza Express alienates those who don't like pizza. Yet, supermarkets are designed to say nothing, do everything, welcome everyone. How do you transform that absence of opinion and personality into exciting, buzzy eating spaces?

Secondly, supermarkets are in a bind: if they employ full kitchen brigades to produce unique menus of ambitious, fresh dishes - which most won't, it's too expensive - that poses the risk of customers making unfavourable comparisons between that gussied-up food, and the ready to eat dishes which they regularly buy from said supermarket. Conversely, if eating out at a supermarket venue is no different, in terms of quality, to a supermarket ready-meal, what's their USP? Who will go out of their way to eat food broadly similar to that which they can buy and microwave off the supermarket shelf?

On the evidence of the Fleet Street Fresh Kitchen, meanwhile, the direct competition that Sainsbury will offer to sandwich shops and takeaways can only start a race to the bottom which may erode many of the last decade's marginal gains in food quality. If that sounds melodramatic, ask yourself this: where would you rather buy a sandwich for lunch, a specialist sandwich shop like Eat or Pret A Manger or Sainsbury's? Or am I underestimating the lure of familiar brand names - has M&S Simply Food improved your life to the extent that you'd want to eat out there?